A linear light polarizing filter allows light having electric field in a certain plane to transmit at a higher rate than light having an orthogonal electric filed. Polarizing filters have been widely used in, for example, ophthalmic products, display devices and optical communication devices. Polarizing ophthalmic lenses are interesting because they have the unique ability to selectively eliminate glare that is reflected from smooth horizontal surfaces like water and ice.
Dichroic materials have been used for the manufacture of light polarizing articles. Dichroic materials, when properly oriented, can preferentially transmit light polarized in a particular direction. Such dichroic material may be polarizing over a relatively wide spectrum, such as the visible spectrum, or they may have the polarization property over a narrow range of wavelength. A group of dichroic material is called pleochroic dyes. A pleochroic dye molecule has varying absorption depending on the orientation of the electric field of the incident light. Some pleochroic dyes are self-orienting when placed on a suitable substrate, others has to be combined with other materials to produce the polarizing effect.
As taught by U.S. Pat. No. 2,400,877, an oriented dichroic dye layer may be formed by depositing them onto a surface that has been gently brushed or rubbed in a single direction. This patent reference also discloses that by stretching a polymeric film, such as cellulose and derivatives thereof, proper orienting force may be obtained for dichroic dyes deposited thereon.
Light polarizing articles, such as polarizing ophthalmic lenses, have been manufactured by using various technologies and materials. Most of these products comprise a light polarizing layer in addition to a non-polarizing substrate. The polarizing layer is mostly formed from a dichroic material mentioned above. The light polarizing products produced and processes for the manufacture thereof have hitherto suffered from various drawbacks.
One problem of the prior art product is the distortion of the optical surface by the polarizing layer in the final product. Some of the light polarizing articles in the prior art are produced by laminating a pre-formed polarizing layer to a substrate. It is difficult, if not impossible, to allow the pre-formed polarizing layer to curve to a perfect fit with a contoured surface of the substrate, as is in the case of ophthalmic products. The less than perfect fit between the polarizing layer and the substrate can lead to undesired cylindrical power in an ophthalmic lens.
A second problem of the prior art process and products is delaminating of the polarizing layer from the substrate during the manufacture process or during the life of the product. This is caused by, for example, (i) a less sufficient adhesion between the substrate and the polarizing layer; or (ii) stress between the layers having differing properties, especially differing thermal expansion coefficient.
Still another problem of the prior art polarizing product and processes for making them involves the leaching of substrate component into the polarizing layer. The leaching, or migration, can cause the deterioration of the mechanical and/or optical quality of the polarizing layer over time.
A fourth problem of the prior art article and process is the hazing of the article. Various causes contribute to this problem. The propagation and expansion of defects, optical and mechanical, on a micro-scale originally, is believed to be the leading culprit.
In addition to the above generally mentioned problems, the specific methods mentioned in the prior art, summarized as follows, have their own unique problems.
Firstly, most of the processes used for the manufacturing of such polarizing articles are based on gluing or embedding organic polarizing films which must be purchased from polarizing film suppliers. For example, WO 0187579 discloses an optical plastic part made by forming a PET polarizing film to a desired contour and incorporating the film into a plastic part before molding. Unfortunately such a process can be carried out only on the lens manufacturing site and not in a prescription laboratory. Moreover films are difficult to be deformed enough to match the curvature radius of high power lenses without optical distortion hence limiting this process to low power lenses.
U.S. Pat. No. 5,286,419 discloses a process for making a light polarizing spectacle lens and U.S. Pat. No. 5,412,505 describes a light polarizing spectacle lens including a lens body and a light polarizing film embedded in the lens body. The lens body is made from a monomer mixture comprising a pre-gelled monomer, conventional monomer and isopropyl carbonate. This process involving the casting of the lens cannot be performed in a prescription lab.
WO 02/073291 describes an optical-quality polarized parts and methods for manufacturing the optical part. The optical polarized part comprises a high impact high optical quality polyurethane construct and a polarizer bonded to the construct. In this process the polarizer is placed in contact with liquid phase polymeric material resulting in a laminate structure. Unfortunately such described processes based on the embedding of polarizing film into the lens body can be used only on the lens manufacturing site.
WO 00/22463 describes a polarizer that is formed of a host matrix and a guest dye. The host matrix is a lyotropic liquid crystal matrix having a predetermined orientation. A guest pleochroic dye is disposed within the host lyotropic liquid crystal matrix. The guest dye is oriented by the orientation of the host matrix. The orientation of the host dye is obtained by applying a sufficient shear to the aqueous dye layer during coating. Such process can be easily used on flat substrate by shear application using a doctor blade process as disclosed in EP 1174738 and WO 02/056066 but cannot be easily used on curved substrate such as lenses.
Another example of polarizing article based on the use of liquid crystal dyes is described in EP 01 71917. This patent describes a transparent laminated polarizing glass articles comprising a support of an inorganic or organic glass, a coating exhibiting polarizing properties applied to one of the surface of the support and an optically transparent polyurethane adhesive laminate having anti-lacerative properties.
U.S. Pat. No. 4,683,153 describes a method for the preparation of a similar transparent laminated polarizing glass article consisting of an inorganic or organic glass, a polarizing coating disposed on one of the surface of the support and a transparent layer adhered to the coating protecting it from humidity. This patent describes a process which consists in first forming microgrooves directly on the surface of the lens by brushing, washing and drying the surface, depositing the organic dyes exhibiting a nematic phase, reducing the water solubility of the dye by ion exchange in an acidic solution, protecting the fixed dye layer by a first coating of aminopropylsilane, contacting the coated lens with an epoxy silane, condensing the silane layers and applying a protective layer to form a barrier layer against humidity.
It is well known that the directors of many nematic liquid crystals tend to assume an orientation parallel to the direction in which an adjacent solid surface has previously rubbed. U.S. Pat. No. 2,400,877 reports that parallel alignment is based primarily on geometric factors rather than detailed molecular forces. Consequently the micro-grooves have to be well defined in order to provide an efficient and reproducible polarization effect. This is particularly difficult because plastic lenses are made of materials having a wide range of hardness hence microgrooves machining parameters must be finely tuned for each substrate having differing composition and/or geometry. This makes the process time consuming and less economical.
More recently, French patent application publication No. 2861852 (application No. FR 03 12686) describes an optical product that includes a polymer layer fixed on the substrate, the polymer layer being grooved in order to align dyes hence providing the polarizing effect. This process that involves grooving an intermediate polymer layer instead of grooving the substrate itself is interesting because polarizing articles can be made from different substrates whatever their intrinsic surface hardness. Unfortunately the article described in this invention suffers of lack of sufficient adhesion between the dye layer and the intermediate polymer layer. Therefore the polarizing articles described exhibit poor durability. Moreover the articles are difficult to make because the dyes layer may delaminate during the manufacturing process. Such delaminating may occur particularly during the steps that involve immersing the lenses in aqueous bath. For example, the dye layer may delaminate at the step where the water-soluble form of the dye is converted into a water-insoluble form by dipping in an aqueous solution of metal salts such as salt of aluminum, zinc, barium or any other metals that are known to give water insoluble form of the polarizing dyes.
Consequently there is a genuine need for a process to prepare a polarizing article overcoming the drawbacks of the prior art.